It is desired in a future hydrogen energy utilizing society to develop a convenient and safe hydrogen storage system using hydrogen storage alloys, from which the hazardous nature of hydrogen explosion is removed. The hydrogen storage alloys are promising for use as a hydrogen fuel container for fuel cell driven vehicles because the hydrogen storage alloy has a higher storing efficiency than the containers storing hydrogen in a gaseous state, and it can prevent accidents resulting from intensive hydrogen gas leakage. When the hydrogen storage alloy is used as a hydrogen fuel container, it is necessary to provide a measuring device detecting residual hydrogen content within the container. It is desirable that the sensor can quantitatively monitor the hydrogen content within the hydrogen storage alloy at a wide temperature range spanning from room temperature to a hydrogen delivery temperature of the hydrogen storage alloy.
One of the conventional methods for detecting hydrogen is to measure hydrogen gas pressure. However, since the hydrogen delivery rate is usually controlled by adjusting the temperature of the hydrogen storage alloy, the hydrogen gas pressure varies widely during operation depending on the temperature. Thus, it is difficult to precisely measure the amount of hydrogen remaining in the storage device by measuring the gas pressure.
Another method is to utilize a flow integrator, which sums up the total gas delivery amount, so that by extracting the total gas delivery amount from initial storage amount, the residual amount is defined. However, this process, seemingly effective, is an indirect process so that it is necessary to use it concomitantly with other direct measuring processes.
Other available techniques for directly measuring the hydrogen content in the hydrogen storage alloy includes a gravimetric technique (measuring total mass of the metal) or measuring the lattice expansion of the alloy metal accompanied by the hydrogenation process. However, since the hydrogen mass is one hundredth lighter than that of the metal, and the lattice expansion is small enough to vary within the elastic limit of the alloy, these methods are not sufficient for quantitative measurement. Other methods such as measuring electric resistance or thermal conductivity of the metal are thinkable, however, these methods are not deemed reliable because they still leave unsolved problems such as fracturing of the alloy due to hydrogenation.
In addition, it is anticipated that the hydrogen fuel container utilizing the hydrogen storage alloy will be, in the future, commercialized as a cassette-type container because of its handling convenience. Therefore, the hydrogen quantity sensor is preferably assembled in the cassette-type container as a fuel hydrogen indicator for indicating hydrogen content within the hydrogen container.